1. Field of the Invention
The present invention relates to an image signal processing system such as a television, a video tape recorder, a video disk player, a digital camera and the like, more particularly, to impulse noise reduction method and circuit which can reduce impulse noise component in the image signal without a damage of the original input image signal.
2. Description of the Prior Art
In such an image signal processing system, for example, a television, a video tape recorder and the like, one of the important problems is to reproduce a clean picture image without noise components and/or other artifacts.
Generally, the picture image provided from broadcasting stations or tape manufacturers, is likely to be mixed with noise due to an image transmitting channel or a repeated copy therefor and further mixed with unnecessary noise depending upon a performance of the iamge signal processing system.
Such that noise is intended to be occurred when an image source originally contains the noise component or a poor film is employed in the image signal processing system. Moreover, the picture image is mixed with the noise in case that a recording is effected under a bad light effect and also the image is affected by a weak electric field during the reception of television image signal.
The noise component is generally divided into impulse noise and line noise and the impulse noise is further divided into white noise and black noise. These noise components may be simultaneously cantained in the image signal. Accordingly, various counterplans must be considered in order to reduce the noise components effectively. Because of that, various technical methods have been developed to reduce the noise component. However, the original image signal is likely to be affected by such the methods.
More particurlary, an examplary one of the methods has been proposed in, for example, U.S Pat. No. 4,926,261 (VIDEO NOISE REDUCTION CIRCUIT) issued on May 15, 1990. With this patent method, a high-pass component is filtered from a brightness signal of the image signal so that the signal is defined and controlled in level to obtain a noise suppressing signal and a low noise output is then obtained by subtracting the noise suppressing signal from the input image signal.
Referring to FIG. 1, there is shown a conventional noise reduction circuit, but similar to that of the patent publication. In the drawing, the brightness signal A shown in FIG. 2 (A), is entered to an input terminal IN in FIG. 1. The input brightness signal A is divided into high-frequency and low-frequency components by means of a low-pass filter (LPF) 2 and a high-pass filter (HPE) 4, as shown in FIGS. 2 (B) and (C). Subsequently, the low-frequency component B from the LPF 2 is amplified by a first amplifier 6 and then entered into an input terminal of an adder 12 while the high-frequency component C from the HPF 4 is input to a coring portion 8 which removes the noise component below a coring level T so as to output a higher signal D than the coring level T, as shown in FIG. 2(D). The output D of the coring portion 8 is amplified by means of a second amplifier 10 and the amplified signal is then entered into the adder 12, as shown in FIG. 2(E). The adder 12 adds the output signals from the first and second amplifiers 6 and 10, and outputs at an output terminal OUT the noise-reduced brightness signal F as is shown in FIG. 2 (F).
With the conventional circuit constructed as mentioned above, however, a detail signal having relatively higher frequency component of the picture image is also removed together with the high frequency noise component, resulting in a problem that the picture image is in a soft condition. That is, in case that pulses of a transition period of the high-frequency image signal shown in FIG. 3 (A) are subjected to process, it is a problem that the image signal is changed into a low-frequency component as shown in FIG. 3 (B), to be thereby outputted a soft picture image.